If you have the graph of the force F as a function of time t, you first need to calculate the area below the graph, which is equal to the product of the average force and the total time:
[tex]\int _{t = 0} ^{\Delta t}Fdt = F_{avg}\Delta t[/tex]

See the attachment for an example. The red graph signfies the average force. The areas below the red graph and the blue graph need to be the same.

Then the force is constant... first find the acceleration of the body. Use this formula:
[tex]V_f = V_0 + at[/tex]
And then when you have the acceleration and the mass it's easy to find the force:
[tex]\Sigma F = ma[/tex]

The force is not necessarily constant Chen, the 'a' (i.e. acceleration) in your first equation is infact the average accelartion (from the data set given it is impossible to deterimne whether a is constant or not).

Chen has alreday given you the correct equations, which can be given as: